Why the dead bug bugs me.

Why the dead bug bugs me.
There are exercises that have a use and become useless or less useful. It always depends - what did your evaluation reveal (screen, test or assess).
Here's why the dead bug bugs me.
The dead bug trains static stability of the trunk with other body parts moving. That might be necessary in individuals with motor control dysfunction in lumbo-pelvic control who need rudimentary static control... But wait....
Many individuals who have lumbo-pelvic dysfunction in the pillar (trunk-hips-shoulders) have unrecognised mobility problems, or pain, that negatively influence motor control - so, the motor control dysfunction is secondary in priority to revealing the presence, or not, of minimum mobility of the lower quarter, trunk and upper quarter, or pain.
For example, an individual whose Modified Thomas Test is positive, or whose FABER test is positive, is one who has demonstrable lower quarter mobility dysfunction - an inability to demonstrate minimum levels of hip extension, hip external rotation and/or sacroiliac/lumbar dysfunction of an as yet non-specific diagnoses. Or an individual who has dysfunctional lumbar-lock thoracic extension/rotation is one for whom further biomechanical testing is required to understand why the trunk is not behaving to minimum acceptable levels.
So, to take a person who has not enough thoracic extension/rotation, hip extension, external rotation and possible lumbo-sacral mobility dysfunction and train them to repetitively activate hip flexors is not to address the primary dysfunction - for when the individual who performs static trunk stability correctly, in supine, with repetitive hip flexion and shoulder movement within a tolerable range, then stands up and is required to use hip extension that they do not have, or hip external rotation that they do not have, in weightbearing, the lumbosacral spine will no longer retain static stability and the exercise will not have transfer....simply because the body will give up stability when it has limited mobility above or below the area of stability.
For that reason, diagnosing pain, mobility and motor control is priority. Then getting a referral for local biomechanical examination and understanding of whether the pain is nociceptive, chemical, central or neuropathic; understanding what movement directions do not have enough mobility - such as lower quarter extension/ER; or thoracic extension/rotation, then improving those and finally retraining the body how to access the new found mobility, which may THEN involve static trunk stability with limb movements.
Where the dead bug falls down, traditionally, as an exercise at THAT point, is that it biases hip flexion activity and not hip extension motor control. The number of individuals with undiagnosed thoracic and hip extension mobility dysfunction, ie below minimum levels of mobility, who are repeatedly activating hip flexion and wondering why they still don't have hip extension is astounding. That's because evaluation reveals what is necessary....the diagnosis before the intervention.
So, a better version is a mobility drill for thoracic extension/rotation, or for hip extension, followed by a motor control for hip extension and/or thoracic extension/rotation - such as prone to supine rolling, quadruped diagonals, half-kneeling lifting, or at lower levels still - the leg-lock bridge, or a core-activated single leg bridge, or if you want to do a dead bug, the lowering leg must finish the movement by actively extending all the way to neutral, and even to 10 degrees or more of hip extension - so a dead bug (if you must), to short-leg or straight-leg bridge.
Assess, don't guess. Don't give people your favourite exercise, give them what objective assessment reveals they need.